Lignocellulosic materials represent a vast amount of renewable resources available in virtually every part of the world. The use of lignocellulosics as a raw material for chemicals continues to be limited by the nature of current delignification processes (i.e., separation of lignin from cellulosic and hemicellulosic components) and by the difficulty of converting the lignin obtained to articles of commerce. This application is directed toward the latter. More particularly, this application is directed to the hydrogenolysis of a species of lignin to afford phenols, especially cresols.
Catalytic hydrogenolysis of lignin was known for some time to effect liquefaction, but its utility was severely curtailed by its tendency to afford a product of little commercial value. With the advent of the so-called Noguchi process (Canadian Pat. No. 700,210) it was claimed that a mixture of C.sub.6 -C.sub.9 monophenols would be obtained upon hydrogenolysis in yields as high as about 40%. The patentee used a catalyst of iron(II) sulfide with a co-catalyst of at least one sulfide of copper, silver, tin, cobalt, chromium, nickel, zinc, or molybdenum, and conducted the reaction in a solvent such as lignin tars and phenols at 250.degree.-450.degree. C. and an initial hydrogen pressure of 150-450 atmospheres. The process was extensively evaluated (David W. Goheen, Lignin Structure and Reactions, American Chemical Society, Advances in Chemistry Series, No. 59) in a multitude of its variants, and although the high yields of monophenols as claimed by the patentee never could be reproduced the investigators concluded that the process remained the best one for lignin liquefaction to that date. However, another conclusion was that the process, even though the best one available, was economically unattractive because of the kind of lignin used, the relatively low economic value of the monophenol product mixture, and the loss of phenol itself when used as a solvent.
More recently research directed toward obtaining useful monomeric products, especially phenolics, from lignin has turned toward the hydrocracking of lignin. Hydrocracking catalysts, used endemically in the petroleum industry, are bifunctional supported metal catalysts having both cracking and hydrogenation activity. Cracking activity generally arises from the support itself, most often through acidic sites, as well as from at least one supported metal, whereas hydrogenation activity usually arises from a supported metal generally recognized as a hydrogenation catalyst. In one of the few published reports directed toward hydrocracking of lignin Hastbacka and Bredenberg, Paperi ja Puu, 55 (3), 1973, used a nickel-molybdenum combination on a silica-alumina support, with most of their efforts directed toward hydrocracking of model compounds rather than lignin itself. Gendler et al., Wood Agric. Residues: Res. Use Feed, Fuels, Chem., Proc. Conf. Feed, Fuels, Chem., Wood Agric. Residues, 1982, J. Soltes, Ed. (Academic Press, 1983), 391-400, have described lignin hydrocracking using an unspecified catalyst in an ebullated bed. Compare R. W. Coughlin et al., Bioconvers. Syst., 1984, D. L. Wise Ed., (CRC), 49-50. The use of CoO--MoO.sub.3 on gamma-alumina in hydrocracking various compounds used as models for lignin was recently described by F. P. Petrocelli and M. T. Klein, Ann. Meeting of American Institute of Chemical Engineers, November, 1984, San Francisco, CA.
We have found a hydrocracking process using a supported nickel-tungsten catalyst which affords phenolics in high yield with little consumption of the phenol used as the pasting oil (lignin solvent). More particularly, when the hydrocracking of lignin is effected with the aforementioned catalyst in the presence of a lower aliphatic alcohol, especially in the presence of both a lower aliphatic alcohol and a Lewis acid which also is a Friedel-Crafts catalyst, and even more preferably in the presence of up to about 25% water as well, good yields of phenols, and especially cresols, can be obtained with high liquefaction of lignin.